Ineffective thermal communication between a heat source and a heat sink hampers the dissipation of excess heat from a system, and damage to system components can occur with ongoing heat buildup due to poor heat transfer. Thermal interface materials (TIMs) can be used in many instances to form a more robust thermal connection between an abutted heat source and heat sink to promote better heat transfer between the two, ideally by minimizing the occurrence of contact voids that diminish the efficiency of heat transfer.
Thermal greases, thermal epoxies, and certain types of metal solder are commonly used for forming thermal interfaces between various structures. The degree to which the heat source and heat sink are mechanically held together depends upon the chosen thermal interface material, with thermal greases providing only weak mechanical coupling and thermal epoxies and metal solders providing stronger bonding. Further, the choice of a particular thermal interface material for a given application can be dictated by the properties of the structures between which thermal communication is to be established. Accordingly, there is no one thermal interface material suitable for universal use across a wide variety of platforms.
Some heat sources and heat sinks are particularly difficult to effectively thermally couple due to chemical or physical incompatibility between the two. For example, coefficient of thermal expansion (CTE) mismatch between the thermal interface material, the heat source and/or the heat sink can result in delamination of the coupled structures. Similarly, if the surfaces of the heat source and the heat sink are of a significantly different chemical nature, ineffective mechanical coupling via the thermal interface material can occur. When the surfaces of the heat source and the heat sink are chemically or physically incompatible with one another, conventional materials can often be insufficient for forming an effective thermal interface, and rise of more costly and labor-intensive materials can often become necessary.
High-power, high-frequency electronic circuits, such as those operating in the microwave-frequency range, are one example of a heat source that can be difficult to effectively thermally couple to a heat sink. For example, monolithic microwave integrated circuits (MMICs) only utilize about 10-50% of their input power to produce an output signal, and the remaining power is expended as considerable amounts of heat. Traditional substrates for constructing MMICs can include, for example, Si, GaAs, InP and SiGe. MMICs fabricated from such traditional substrates can usually be bonded to a heat sink using thermal epoxies, such as silver epoxy.
Thermally stable substrates such as GaN and SiC, for example, can enable much higher MMIC operating temperatures. Thermal epoxies can only be used for these types of MMICS when they are operating at less than their maximum possible input power, so as to limit the amount of output heat. For the high, operating temperatures characteristic of full or near-full power-operations, thermal epoxies generally possess inadequate temperature stability, and more robust thermal interface materials such, as AuSn solder, for example, are often used for establishing a thermal interface. AuSn solder, however, is expensive, requires laborious application and curing conditions, and is prone to void formation when flowed. High-power light-emitting diodes (LEDs) can present similar issues to those encountered when working with MMICs. Moreover, intermetallic compound formation can take place during processing of AuSn solder, producing brittle phases such as AuSn4, which can lead to mechanical failure during operation due to factors such as vibration/shock and thermal cycling.
FIGS. 1A and 1B show illustrative images of voids 1 that can commonly occur during the use of AuSn solder 2. FIG. 2 shows a corresponding x-ray image of similar voids that can occur when using AuSn solder, demonstrating that the voids extend in both dimensions in the x-y plane. The voids can decrease the degree of thermal communication that takes place through the thermal interface layer.
In addition to the foregoing difficulties, CTE mismatch usually necessitates the use of specialized copper alloy (e.g., W/Cu or Mo/Cu) heat sinks to match the CTE of the high-temperature MMIC in order to decrease the likelihood of delamination. Such copper alloy substrates are prone to formation of a passivating metal oxide surface layer, which can further complicate the working conditions needed to establish an effective thermal interface layer.
The U.S. Government is actively seeking new means through which the challenges associated with MMICs and other high-power electronic devices can be better addressed, particularly the elimination of AuSn solder in their manufacturing. In view of the foregoing, improved thermal interface layers within device assemblies containing high-power electronic components, such as MMICs and LEDs, and methods for their production, are of considerable interest in the art. The present disclosure satisfies the foregoing need and provides related advantages as well.